PresencePushTask Class

An XMPP helper class that monitors for presence notifications sent by
the server. When they are received, they send a signal with the status
information wrapped in a class. To receive these messages, connect to
the SignalStatusUpdate signal. You should use this class
to monitor when another user goes offline. If the client has a session
with a user that has gone offline without sending a termination message,
you can either send a notification to the user ("Jim has gone offline")
or end the session. The methods shown are not thread-safe, and can be
called on any thread, but you should call them from the signaling thread.